Conventionally, wireless receiving circuits used in wireless portable devices (e.g. mobile phones, portable telereceivers, and the like) have been primarily those of a heterodyne architecture. In recent years, receiving circuits of a direct conversion architecture or a low IF (Intermediate Frequency) architecture are also used. A receiving circuit referred to here is a so-called tuner, which is a circuit for picking up a signal of a desired channel from all high-frequency signals received by an antenna and converting the picked up signal into a baseband signal having a sufficient amplitude.
A receiving circuit of the heterodyne architecture is advantageous in that it can remove an interfering wave other than a desired channel almost completely by (i) an intermediate frequency circuit having one or two stages and (ii) filters provided on the upstream side and downstream side of the intermediate frequency circuit. A receiving circuit of the heterodyne architecture having a one-stage intermediate frequency circuit is called a single-conversion-method receiving circuit, and a receiving circuit of the heterodyne architecture having a two-stage intermediate frequency circuit is called a double-conversion (or dual-conversion)-method receiving circuit.
On the other hand, a receiving circuit of the direct conversion architecture can hardly remove the interfering wave, and the interfering wave is inputted to a baseband circuit. Therefore, the receiving circuit of the direct conversion architecture requires a filter, provided on the upstream side of the baseband circuit, that has a steep attenuation characteristic and a saturation-resistant characteristic for sufficiently attenuating a high-power interfering wave. The merit of the receiving circuit of the direct conversion architecture is its low power consumption owing to its simple circuit arrangement. Because of this merit, the receiving circuit of the direct conversion architecture is used as a wireless receiving circuit for the wireless portable devices.
Although a receiving circuit of the low IF architecture uses the single conversion method, it has almost the same features as the receiving circuit of the direct conversion architecture. This is because an intermediate frequency is a low frequency about several times higher than a baseband signal range. Therefore, in the following, a receiving circuit of the low IF architecture is implicitly included in the receiving circuit of the direct conversion architecture, unless otherwise noted. Hence, in the following, a baseband circuit includes an intermediate frequency circuit of a low frequency several times higher than the baseband signal range, unless otherwise noted.
Conventionally, in the receiving circuit of the direct conversion architecture, a filter provided on the upstream side of the baseband circuit is made by combining a plurality of receiving elements, such as a passive low-pass filter (including an inductor and a capacitor, or a resister and a capacitor) and an active low-pass filter (a transistor or the like) (see, for example, International Patent Publication WO 00/51251, published on Aug. 31, 2000, pp. 16-21 and FIGS. 12-19; also published as EP1081871A1).
Specifically, as shown in FIG. 6, this receiving circuit includes an antenna 41, a band pass filter 42, a low-noise amplifier 43, two mixers 45, 46, a voltage control oscillator (VCO) 44, and a baseband circuit 51. The antenna 41 receives high-frequency wireless signals. The band pass filter 42 allows passage of only a signal in a predetermined frequency range required by the receiving circuit, among the high-frequency wireless signals received by the antenna 41. The low-noise amplifier 43 amplifies the signal that has passed through the pass filter 42. The mixers 45, 46 mix the output signal of the low-noise amplifier 43 with signals at a local oscillation frequency (local oscillation signals), so as to downconvert the output signal of the low-noise amplifier 43 into baseband signals I and Q. The VCO 44 generates two signals having a 90-degree phase difference at a frequency equal to the center frequency of the desired channel, and supplies these signals to the two mixers 45, 46, as signals of the local oscillation frequency. The baseband circuit 51 processes the baseband signals I and Q.
Two kinds of passive low-pass filters (passive low-pass filters 59, 60 and passive low-pass filters 61, 62) and one kind of active low-pass filters 49, 50 are provided on the upstream side of the baseband circuit 51, that is, between the mixers and the base band circuit 51. Among the two kinds of passive low-pass filters 59, 60, and 61, 62, the passive low-pass filters 59, 60 remove an interfering wave at a frequency higher than adjacent channels (channels that are adjacent to the desired channel). The passive low-pass filters 61, 62 remove a high-frequency interfering wave to which the active low-pass filters 49, 50, which are provided as a next stage, will be ineffective (that is, a high-frequency interfering wave that will not be removed by the low-pass filters 49, 50). The low-pass filters 49, 50 remove interfering waves in the adjacent channels.
If a receiving circuit is ultimately provided on a single chip, that is, if the receiving circuit is provided on the same substrate as an IC, the receiving circuit occupies only a small area, and costs for parts become lower. Therefore, it is desired to provide the receiving circuit of portable wireless devices, such as mobile phones and portable telereceivers, on a single chip. To realize this, power consumption of the receiving circuit must be low, because the portable wireless devices must function with internal batteries only.
Therefore, an object of the present invention is to provide main circuits of the receiving circuit, which is of the direct conversion architecture or the low IF architecture, on about one or two IC chips, and for this purpose, to provide low-pass filters within the receiving circuit, on the upstream side of a baseband circuit or a low/intermediate frequency circuit, without involving additional power consumption by the filters. In reality, it will not be attained soon to provide on one chip the entire receiving circuit, including such members as an antenna and a bypass capacitor, which prevents (i) voltage fluctuation between a power source and a ground and (ii) mixture of noises. Therefore, the present invention aims at providing main circuits on one or two IC chips.
In integrating the receiving circuit of FIG. 6 into an IC, there are problems with the low-pass filters 59 to 62, which include capacitors and inductors. Even if a cutoff frequency of these filters is about 10 MHz, the inductance of the inductors are about several μH if the capacitances of the capacitors are several dozen pF. It is nearly impossible to provide the inductors having such inductance on the same IC substrate as other elements.
To solve this problem, one conceivable way is to replace the inductors by resistors, which can attain relatively a wide range of values (resistance values) within an IC, and to realize the low-pass filters by resisters and capacitors.
However, while ideally the low-pass filters realized by inductors and capacitors do not cause a loss of electric power, the low-pass filters realized by resistors and capacitors cause a loss of electric power, because of the resistors. In some causes, it can be assumed that the signal source for the low-pass filters is only voltage or current. Even in such cases, there is a loss of signal voltage or signal current, because of the resistors. Therefore, it is necessary to consider the loss of signal power, voltage, or current caused by inserting the low-pass filters including resistors and capacitors (so-called insertion loss), in designing not only the low-pass filters but also circuits provided on the upstream side and downstream side of the low-pass filters. As a result, a new amplifier circuit is required. Moreover, because of the loss of signal power, voltage, or current, it is necessary, in designing the receiving circuit, to consider impedance matching between input and output impedance of the low-pass filters and the impedance of the circuits provided on the upstream side and downstream side of the low-pass filters. Since the filters do not include inductors, the impedance matching cannot be performed by adjusting inductors. Therefore, for the purpose of impedance matching, it is necessary to provide buffers such as active impedance conversion circuits (e.g. emitter follower circuits, source follower circuits, and the like) on the upstream side and downstream side of the low-pass filters. The addition of the amplifier circuit and the buffers leads to increase of power consumption.
Thus, if the receiving circuit includes high order passive low-pass filters realized by resistors and capacitors, it is difficult to reduce power consumption of the receiving circuit and to integrate the receiving circuit into an IC. In addition, it is very difficult to design the passive low-pass filters and the circuits provided on the upstream side and downstream side of the low-pass filters.